Expulsion fuse



June 12, 1934. B. R. HERMANN EXPULSION FUSE Filed Feb. 27, 1932 atented June 12, 1934 EXPULSION FUSE Benjamin R. Hermann, Pittsfield, Mass., assignor to General Electric Company, a corporation of New York Application lFebruary 27, 1932, Serial No. 595,493

3 Claims.

The present invention relates to electrical protective devices and more particularly to expulsion fuses.

Expulsion fuses are designed to utilize the gases 5 generated by the action of the arc upon the surrounding material in preventing that arc from reforming after it has reached a period when the current is practically zero, which happens twice in every cycle of an alternating current. When these gases arising from the decompositionv of the walls of the fuse tube and vaporizing of the fuse metal, are at a high pressure or densely concentrated inthe arc path, the arc can not easily restrike and the ow of fault currentA it is, therefore, readily understood that ceases the ability of the fuse to function `pr'operly is inherently connected!" with the size of the arc chamber and its proximity to the arc, as well as with the ferocity of the arc itself. ber is made so small as to give gas densities necessary to interrupt small arcs of relatively low gas making ability, it is then so small as to be easily burst by the tremendous pressures built up in it by an arc of -greater power. It has been necessary in the past to .compromise onthe size of the chamber by making it sufliciently large so that execessively high gas` pressures would not be generated by large fault currents and thus sacrifice entirely satisfactory op- 30.A eration on relatively small currents such as socalled overloads.

1f the cham ing range of the fuse cutout.

Another object of my invention is to provide an improved renewable fuse link which is rugged in construction and will thus withstand hard usage by inexperienced hands.

What I consider to be novel and my invention will be better understood by reference to the following specication and appended claims when considered in connection with the accompanying drawing.

In the drawing, Fig. 1 is a sectional view of an expulsion fuse unitconsisting of a conventional type of fuse holder and a renewable fuse link of the new design which increases the operating range. Fig. 2 is a sectional view of the fuse link.

Referring to the drawing, Fig. 1, the casing or fuse holder of the expulsion `fuse unit consists of a tube 10 of vinsulating material of relatively On the lower end of tube 10 is mounted terminal contact 11 surrounding the 'tube which is spaced from the end of it. The other terminal contact 12 is secured on the upper end of tube 10 and, as shown, forms a chambered extension of the tube with the chamber communicating with the'bore of the tube. rl'erminal contact 1l has a lower extension 13 which is threaded for a portion of its length and terminates in a tapered end provided with a ribbed surface. A nut 14 is adapted to engage the threads of the extension 13 and is provided with a tapered seat to cooperate with the end of the extension. The terminal of fuse link 15 is adapted to be held in engagement between the tapered seat of nut 14 and the ribbed tapered end of extension' i3 to connect the fuse link 15 to contact 11. Contact 12 has an upward cylindrical extension 16 which is threaded at its lower portion for engagement with a thimble or nut 17 which closes the end of the extension and is adapted to clamp the button head 18 of fuse link 15 in engagement with the end of extension 16 thus connecting the other end of fuse link 15 to contact l2. The explosion chamber of the fuse consists of the central bore of tube 10 and the chamber' formed by contact 12 and its extension 16. This chamber is closed at one end by the nut 17. The fuse link 15 is supported in the explosion chamber of the above described type of fuse holder by means of the button or metal disk 18-at the closed end of the chamber. The other terminal of the fuse link 15 lextends through the" open end of the chamber and is connected to contact 11. The walls of tube 10 must be of sufficient thickness and of tough material to withstand the gaspressure generated in the chamber on the occurrence of a fault with heavy current flow.

Fuse link 15 consists of a fusible element 19 enclosed in a thin walled tube 20 of insulating material. Fusible element 19 may be of tin or other fusible metals or alloys either in wire or strip form, so shaped that the blowing point is located near the upper lend of tube 20. Tube 20 is closed at one end by a stopper 21 which is cemented in position 'in the end of the tube to seal the end. Stopper 22 is placed in the bottom of tube 20 with a snug iit to permit gas pressure to build up within the tube when the fuse link is blown by a light ciirrent. However, thisstopper is not indispensable and may be omitted. Fusible element 19 is connected by means of a hard metal wire 23 which passes through stopl of tube 20. By this arrangement if the fuse link is to be used with a fuse holder of a different type requiring two wire fuse terminals the wire 23 may be cut near the button 18. and the remaining portion of Wire 23 will be sufficiently long to make the connection to the contact of the fuse holder. The other end of fusible element 19 is connected to terminal wire 24 which passes through stopper 22. The tube 20 is provided to obtain an auxiliary explosion chamber surrounding the fusible element 19 small enough to produce the necessary gas pressure for the operation of the fuse on light currents. When a heavy current fault occurs the extremely high gas pressure generated bursts the thin walled tube 20 and permits the gas to expand within the larger explosion chamber thus reducing the gas pressure to a safe value and furnishing the normal arc extinguishing, expulsion action of the larger chamber. Thus the combination of small tube 2O of the fuse link 15 with the larger tube 10 produces an expulsion fuse which has improved operating characteristics on small currents and yet operates with equally satisfactory characteristics on high short circuit current values.

The operation of my improved expulsion fuse under different conditions of overload is as follows: At currents of low value, for example currents of the order of 5 to2() amperes, in the case of a one ampere fuse, the action is that of the simple cartridge type fuse link. The soft metal or fuse wire melts and the arc, being of very low stability in this current range, is extinguished before enough gas pressure is built up to blow out the bottom stopper. Of course the lower limit of the operation of the fuse unit depends upon the size of fuse wire which in turn is determined by the desired rating of the fuse link. At currents of intermediate values, for example currents of the order of 20 to 300 amperes the action of the arc builds up a pressure which blows out the bottom stopper. The turbulent blast of gases which are held in a relatively high pressure condition by the auxiliary tube walls quickly extinguishes the arc. It is in this range that the auxiliary explosion chamber comes into action to improve the operation of the expulsion fuse. Without this smaller tube the gas pressure would not be suicient to prevent the arc from re-establishing after every period of zero current. f

At currents of high value, for example currents above 300 amperes, the great heat of the arc generates a high pressure which shatters the small tube and expels it with the fuse wire from the larger tube quickly extinguishing the arc. The expansion of the gas on the shattering of the small tube reduces its pressure within the explosion chamber of the fuse holder sufliciently so that no damage is done to the larger tube unless, of course, the interrupting capacity of the fuse cutout is exceeded.

The fuse link 15 with the soft metal or fuse wire 19 completely enclosed within tube A2G in addition to being a fuse link of improved operating characteristics is of very sturdy construction and will withstand hard usage. The stoppers 21 and 22 in each end of the tube 20 relieve the fuse wire 19 from any tensile strain whichI may be communicated through the hard metal wires 23 and 24 and the tube of insulating material protects the delicate fuse wire from mechanical injury. This is an important feature of my invention because it frequently happens in actual service that a blown fuse link will have to be replaced by one who is not experienced in the replacement operation and so will not handle the fuse links as carefully as former fuses required. The use of hard metal wires 23 and 24 and the confining ofthe fuse wire 19 within the tube 20 prevents atmospheric corrosion from affecting the predetermined rating of the fuse link.

While in the foregoing specification, I have described a particular embodiment of my invention, it will be apparent to those skilled in the art that changes may be made Without departing from my invention and I, therefore, aim in the appended claims to cover all such changes that fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is,-

l. A fuse structure comprising walls which dene a primary fuse chamber, walls which define an auxiliary fuse cli'amber within the primary fuse chamber, an opening in a wall of the auxiliary chamber closed by a stopper, a fusible member in the auxiliary chamber, and terminals for the fusible member, the stopper snugly fitting in the opening and adapted to blow out of the opening on a relatively low current fault, and the walls of the auxiliary chamber being shattered on a heavy current fault within the interrupting capacity of the fuse.

2. A fuse structure comprising a tube closed at one end forming a primary fuse chamber, a second tube secured in said first tube, Stoppers closing each end of said second mentioned tube, a fusible element secured in said second mentioned tube whereby a stopper is adapted to blow out of said second mentioned tube on low current faults and said second mentioned tube is adapted to blow out of said first mentioned tube on heavy current faults.

3. A fuse comprising a thick walled tube, means closing one end of said tube, a thin walled tube located in said thick walled tube, a fusible element located in said thin walled tube, and stoppers closing said thin walled tube whereby a stopper is adapted to blow out of said thin walled tube on low current faults and the thin walled tube adapted to blow out of said thick Walled .3@ 

